perf(ui): stop animating BackdropFilter blur sigma in dialogs (Fluid F3)

[ZhuchkaTriplesix]

Parent

Part of #356 · Fluid perf audit F3 (P0)

Problem

showAppDialog animates ImageFilter.blur(sigmaX: 8.0 * t, …) inside AnimatedBuilder every frame. Changing blur sigma is raster-heavy and fights the 120 Hz frame budget.

Where: lib/shared/widgets/app_dialog.dart

Expected

• Blur is static (or pre-blurred) during open/close
• Dim / opacity (and card fade-slide) carry the enter/exit motion
• Motion Off / Reduced still honored

Acceptance

• No per-frame blur-sigma animation
• Enter/exit still use QueryaMotion enter/exit curves on the card
• Widget/golden or dialog tests updated if needed
• Manual: open/close Preferences or connection dialog — no jank spike from blur

Related

• Epic Fluid: epic(ui): Fluid morphing — evolve QueryaMotion (post-0.4.11 shell) #342
• Checklist: docs/perf-baseline.md §13 Dialog / dropdown

[ZhuchkaTriplesix]

Fixed and merged via PRs #367/#368/#369 on dev.